The experience of application self-propelled percussive action machines for driving holes in the ground has shown that they are very efficient and simple to operate. It has also been found that the reliable and accurate hole driving depends largely on the properties of the soil in which the hole is made. When a non-homogenious soil or a soil which has voids, rock inclusions, and construction site leftovers are encountered, the housing of such machines may be susceptible to bending to result in a curvilinear path of travel of the machines, and therefore a straight-line holes are difficult if not impossible to obtain. High bending loads exertable on the housing of the machine may cause its failure due to jamming of the hammer inside the housing.
There is known a machine for driving holes in the ground (cf., USSR Inventor's Certificate No. 658,224; published 1979) comprising a bore rod, a cylindrical calibrating portion, a tapered head end section, and a finned conical sleeve interposed between the calibrating and head end sections. The head end section is separated from the conical sleeve by a cylindrical section. The arrangement of the machine with two tapered surfaces axially spaced apart is as advantageous as one tapered section having a maximum diameter equal to the maximum diameter of the ribbed finned conical sleeve. In other words, the machine may be viewed as one having a single tapered section which, during an encounter with an obstacle of the head end section thereof will tend to deviate from the rectilinear forward path to result in a curved hole.
There is also known an apparatus for driving holes in the ground as exemplified in U.S. Pat. No. 4,144,941, published Mar. 20, 1979. This machine comprises a cylindrical housing having disposed therein a hammer capable of delivering impacts on a head end section of the housing and an air distribution mechanism. The head end section of the housing has a diameter greater than the diameter of the rest of the housing.
This machine is charaterized by a relatively small contact area between the walls of the housing and the ground, which results in reduction of the forces taking up the recoil of the housing during acceleration of the hammer to therefore slow down the rate of hole driving or even to terminate the travel of the housing, expecially in soils which are not compact or in moist soils. In addition, the thickened part of the housing acts to reduce the energy of impact per unit area of the hole being made to eventually slow down the rate of hole driving.
The thickened part of the head end section of the housing increases the overall mass thereof to result in less efficient transmission of impacts to the soil, which also leads to slowing down the rate of hole driving.
Further, the small length of the thickened part of the housing in contact with the ground fails to assure directional stability during the forward percussive travel of the machine in the ground, especially when the thickened conical portion of the housing encounter an obstacle, such as a boulder.
There is further known a machine for driving holes in the ground (cf., U.S. Pat. No. 4,132,277, published Jan. 2, 1979) comprising a cylindrical housing, a hammer disposed inside the housing for delivering impacts on a head end thereof, and an air distribution mechanism.
The above machine features a relatively thin-walled housing, which is favourable from the point of view of attaining a maximum impact power and maximizing the efficiency of transmission of impacts to the ground thanks to a relatively small mass of the housing.
The machine is also characterized by a sufficiently extensive length of the housing to result in a greater accuracy of the forward travel path of the machine.
However, the elongated housing with thin walls is susceptable to bending when travelling through non-uniform soils, as well as soils having voids and rock inclusion, which may cause the machine to deviate from the preset course.
Further, very pronounced bending of the housing cause operating failures due to jamming of the hammer.